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UNC-Chapel Hill GEOG 192 - Lecture 2 – Spatial Data Preparation

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Lecture 2 – Spatial Data Preparation2-1 Scale2-2 Map projection2-2 Map projection (Cont.)2-2 Map projection (Cont.)Slide 62-3 Geo-reference2-3 Geo-reference (Cont.)Slide 9Slide 102-4 Data Conversion19/1/1419/1/14Jun Liang, Geography @ UNCJun Liang, Geography @ UNC11Lecture 2 – Spatial Data PreparationLecture 2 – Spatial Data PreparationBefore we can create a GIS project, we need to collect and process data, which including spatial data, and attribute data.Several important issues during this process:(1) Scale(2) Map projection(3) Geo-reference(4) Data conversion19/1/1419/1/14Jun Liang, Geography @ UNCJun Liang, Geography @ UNC222-1 Scale2-1 ScaleSpatial resolution is critical to GIS application.Several factors you may consider when you decide your spatial data resolution:(1) Your research area(2) Other data sets(3) Software and hardwareExample:Hydrology modeling input layers – land-use map, remote sensing images, and climatic factors.19/1/1419/1/14Jun Liang, Geography @ UNCJun Liang, Geography @ UNC33 2-2 Map projection2-2 Map projectionDifferent ways of projecting could be produced by surrounding the globe in a cylindrical fashion, as a cone, or even as a flat piece of paper. Projections onto these three surfaces are called c ylindrical, conic, and azimuthal, respectively.Note: No map projection can preserve both conformality and equal area.19/1/1419/1/14Jun Liang, Geography @ UNCJun Liang, Geography @ UNC44 2-2 Map projection (Cont.)2-2 Map projection (Cont.)Two major projection steps:(1) Select a reference globe – converts the actual globe to a reference globe.(2) Select a projection – the reference globe is mathematically projected onto the flat surface.Datum (ellipsoid) – Clark1866/NAD27(North American Datum), NAD83/GRS80 (Geodetic reference system), WGS84 (world geodetic system, a modified version of GRS80)19/1/1419/1/14Jun Liang, Geography @ UNCJun Liang, Geography @ UNC552-2 Map projection (Cont.)2-2 Map projection (Cont.)The most common coordinates systems for USA:(1) UTM – Universal Transverse MercatorUTM, which has been included since the late 1950s on most USGS topographic maps. UTM is often used for accurate mapping.(2) SPC – State Plane Coordinate SystemState plan coordinate system (SPC) was devised by the US Coast and Geodetic Survey in 1930S. SPC uses either a transverse Mercator or Lambert’s conformal conic projection.19/1/1419/1/14Jun Liang, Geography @ UNCJun Liang, Geography @ UNC662-2 Map projection (Cont.)2-2 Map projection (Cont.)Map projection conversionParameters you map need for converting map projection A to map projection B:(1) Coordinate systems (UTM, SPC, World, Gauss Kruger, etc.)(2) Datum (Nad27, Nad83, Nad83-feet, etc.)(3) Projection Zone (OHIO north, OHIO south, North Carolina, etc.)(4) Standard latitude/longitude lines(5) Projection19/1/1419/1/14Jun Liang, Geography @ UNCJun Liang, Geography @ UNC772-3 Geo-reference2-3 Geo-referenceGeoreferencing is the process of scaling, rotating, translating and transforming the spatial data to match a particular size and position, or a controlled coordinate system. Why we need geo-referencing spatial data?-Old maps (projection and origins unknown and can not be retrieved.-Aerial photos-Scanned maps19/1/1419/1/14Jun Liang, Geography @ UNCJun Liang, Geography @ UNC882-3 Geo-reference (Cont.)2-3 Geo-reference (Cont.)Geo-referencing Raster MapRaster image – made up of pixels; no particular size. Without projection coordinate system information (cell size, projection, coordinates, etc.), it will not be displayed correctly in GIS software.To register a raster image to a specific coordinate system, you will need to choose a projection for your data frame that will most closely resemble the geometry of your map. GCPs (Ground Control Point) should also carefully selected to register aerial photos.19/1/1419/1/14Jun Liang, Geography @ UNCJun Liang, Geography @ UNC992-3 Geo-reference (Cont.)2-3 Geo-reference (Cont.)Vector data can also be geo-referenced. (when?)Steps of geo-referencing in ArcGIS:There are two ways you can geo-reference your raster image: (1) by assigning coordinates that you type in as taken from a paper map or (2) by aligning it with a target layer that has a coordinate system. General steps:1. Add the raster image (and any target layers you want to align it to.)2. Add control points that link known raster positions to know target data positions in map coordinates3. Save the geo-referencing information with the raster image as an .aux file19/1/1419/1/14Jun Liang, Geography @ UNCJun Liang, Geography @ UNC10102-3 Geo-reference (Cont.)2-3 Geo-reference (Cont.)Transform/WarpWhen you created enough GCP links, you can transform your raster dataset to permanently match the map coordinates of the target data. It uses a polynomial transformation to determine the correct map coordinate location for each cell in the raster.More GCPs can increase the overall accuracy of the transformation.GCPs’ quality are also important.19/1/1419/1/14Jun Liang, Geography @ UNCJun Liang, Geography @ UNC11112-4 Data Conversion2-4 Data Conversion1. Raster to Raster2. Raster to Vector3. Vector to VectorExamples: jpg->tiff, img->jpg/tiff, img->grdgrd->shapefile,


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UNC-Chapel Hill GEOG 192 - Lecture 2 – Spatial Data Preparation

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